AERIAL PHOTOGRAPHY AND FORESTRY IN ASIA
filled, then there will be a fair chance that the
results obtained will be likely to give disap-
pointments; a result, which can only be to the
detriment of the good reputation of aerial
photography.
But even if working conditions are optimal,
the final interpretation results are also largely
determined by the composition of the vegeta-
tion to be studied. For instance interpretation
possibilities upon aerial photographs will be
entirely different for tropical rain forests com-
pared with the same procedure in the Mon-
soon forests, as, within the latter vegetation
form, much more differentiation in the type
components can be traced. It will be in this
special context that reference is made to a
former paper in which the author discussed
this problem in greater detail. (3).
Large parts of S. E. Asia are situated in the
typical monsoon region, including a large part
of India and Ceylon and further the main
parts of Burma, Thailand and Vietnam. Real-
ization of this will make clear, that, in these
countries, the possibilities for photo-interpre-
tation are favorable. Also knowing that one
of the most frequently occurring tree spe-
cies in the monsoon forest is the teak tree
(Tectona grandis)—a tree species which is
highly appreciated in the world timber mar-
ket—there will be no doubt of the great bene-
fits to be derived from the application of
aerial photography, as a method for the com-
pilation of a forest inventory which can be
fully justified economically in those countries.
METHODS OF PROCEDURE
To get more information regarding the
manner in which aerial photographs can be
applied in forest inventory, a short explana-
tion of general methods of procedure must be
given. Normally the following items are of
special interest:
A. AVAILABLE PHOTOGRAPHICAL MATERIAL
AND SPECIAL REQUIREMENTS
In most cases the necessary photographic
material can be obtained from the local topo-
graphic mapping departments or Army map-
ping services. Big parts of the different coun-
tries in S. E. Asia are already covered by
aerial photographs, and the map production
in general is keeping pace with the area cov-
ered by the photographs. The most appropri-
ate photo-scale for good interpretation results
is therefore mostly dependent upon the spe-
cial requirements of the mapping depart-
ments. For that reason photo-scales in general
are rather small, mostly varying between
1:25,000 and 1:50,000. Although foresters
generally prefer larger scales for forest photo-
interpretation, these smaller scales will give
quite reasonable results as they permit ob-
taining a better overall impression of the dis-
tribution of occasional type differences within
one stereoscopic model. Furthermore photo-
graphs are mostly made in the first part of the
dry season, when flying conditions are opti-
mal. This makes the conditions for photo-
interpretation as favorable as possible, for the
chances of the photo-image being blurred by
occasional dust in the atmosphere, or by
smoke from grass burning, will be relatively
low.
Mostly panchromatic film is used in combi-
nation with a yellow filter, which film-filter
combination is, besides the green colors, also
sensitive for yellow and red colors, which can
also be favorable for good interpretation con-
ditions. While infrared film does not seem to
have been applied so far in S. E. Asia, it ap-
pears worth-while to make some tests with
this kind of film in this part of the tropics, as
results obtained in other regions are rather
encouraging.
B. PRELIMINARY OBSERVATIONS IN THE FIELD
Before starting with the actual photo-in-
terpretation, it will be advisable to make some
preliminary observations in the field. There-
fore it will be necessary to obtain a general
knowledge about the occurrence of main vege-
tation types. This knowledge is mostly ob-
tainable from available literature or by in-
formation obtained from local forest service.
Based on this knowledge a general reconnais-
sance of the occurrence of these types in the
field could be made. During this trip compari-
sons of the terrain features with the corre-
sponding photo-images could be made for the
purpose of ascertaining if the different vegeta-
tion types based on terrestrial studies are also
identifiable on the photographs, or con-
versely, if type differences located on the
photographs are an indication of particular
vegetation types, so far unknown by terres-
trial survey methods.
During this reconnaissance work there will
be a good opportunity to make stereograms of
the prevailing vegetation types to be used as a
reference for the interpretation afterwards in
the office.
C. PHOTO-INTERPRETATION
Based on his information, obtained during
his reconnaissance, the interpreter will be able
to differentiate the various kinds of vegeta-
tion types under his stereoscope. These type
differences can be a result of natural circum-
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